It has become common practice in recent years to provide an exhaust gas recirculation (EGR) system in an internal combustion engine, in order to reduce the emission of noxious gases. Such systems have been included in turbocharged engines, and U.S. Pat. Nos. 3,149,454, 3,914,944 and 3,925,989 show examples of such arrangements. Systems of this character have included an EGR line connecting the engine exhaust manifold with the intake manifold.
For such an EGR system to work in a turbocharged engine where the EGR line is connected between the exhaust and intake manifolds, the pressure in the exhaust manifold must be higher than the pressure in the intake manifold in order to produce the desired flow toward the intake manifold. This means that high efficiency turbochargers which are capable of operating with the exhaust manifold pressure lower than intake manifold pressure cannot be used and the engine fuel comsumption is thereby compromised.
An engine may be constructed including an EGR line connected to receive exhaust gases from less than all of the cylinders (EGR pumping cylinder), but this may also prove to be disadvantageous because the exhaust gases may not be recirculated and evenly distributed to the intakes of all of the cylinders. For example, if the entire exhaust flow from one cylinder were recirculated, the one or two cylinders whose intake strokes are next in the firing order would receive most of the recirculated gas.
Another disadvantage of the proposal whereby the exhaust gases from a group of cylinders is directed in part to the intake manifold with the remaining part directed to the turbocharger turbine is that, to make the exhaust manifold pressure greater than the intake manifold pressure, it is proposed to place a valve or orifice in the duct from the EGR pumping cylinders to the turbocharger in order to increase the back pressure on the EGR pumping cylinders and produce the desired EGR rate. Such an arrangement has the disadvantage of increasing the piston pumping work penalty of the EGR pumping cylinders, and thereby producing energy losses.
It is a general object of the present invention to eliminate the foregoing problems by providing an engine including improved EGR system.